Everyone has a favorite rdbms, right? Mine's PostgreSQL.
"SQL is a domain-specific language used in programming and designed for managing data held in a relational database management system (RDBMS), or for stream processing in a relational data stream management system (RDSMS). It is particularly useful in handling structured data, i.e. data incorporating relations among entities and variables."
I used SQL in the projects below in some way or another. Keep in mind that this is just a selection.